Chemical processing of cellulose pulps



Patented July 7, 1931 UNITED STATES PATENT orrlcs GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE; ASSIGNOR TO BROWN COMPANY,

OF BERLIN, NEW HAMI'SHIRE, A CORPORATION OF MAINE CHEMICAL raocsssme or cELLULo sE runes No Drawing.

This invenion relates to the chemical-processing of cellulose pulps to improve their quality, and, while not limited thereto, is more especially concerned with the chemical processing of those types of wood pulp produced, by alkaline cooking processes such as the kraft or the soda process.

There are various processes of liberating cellulose fiber from raw cellulosic materials such as wood, each of which yields apulp having more or less well-defined physical and chemical characteristics, even though similar raw cellulosic materials be used in the various processes. For instance, wood pulp produced by the acid sulphite process in which the cooking liquor consists of a sulphurous acid solution ofa sulphite is distinctly different physically and chemically from wood pulp produced by the kraft process in which the cooking or kraft liquor consists of a solution of caustic sod a" and sodium sulphide, even though spruce chips constitute the raw cellulosic material! The differences between the various types of pulp may be ascribed to the difference in action of the cooking liquors employed upon raw cellulosic material, an acid-sulphite liquor, for instance, being more eflective in its action upon ligneous matter and pento'sans associated with raw cellulosic material such as wood, than alkaline liquor such as kraft liquor, with the result that sulphite pulp has a lower ligneous and pentosan content and is lighter in color than kraft pulp. Sulphite pulp may therefore be readily leached towhiteness, whereas the bleaching of kraft pulp is accompanied by much higher wood content and yet having approximately the Application filed April 15, 1929. Serial No. 355,428

physical characteristics, including strength,

and tear resistance, of the original kraftor soda pulp. In my application-Sen No. 40,959, filed July 1, 1925, this object is attained by digesting the kraft or soda pulp in an acid-sulphite liquor having a combined S0 content higher than its free S0 content, and then bleaching the digested fiber. It is stated in that application that a sulphite liquor containing more combined than free S0 is capable of removing a substantial proportion of impurities, especially pentosans, from pulp such. as kraft, and that kraft pulp when digested in such liquor'may .bebleached to a whiteness of the usual bleached sulphite pulp without being depreciated in its physical characteristics. It is further stated in that application that when digestion ofsuchpulp is effected in an acid-sulphite liquor containing more free than combined $0 the resulting pulp is degraded physically and its strength and tear resistance are considerably lower than that of the original pulp. The resulting pulp is also lower inalpha cellulose content than the original pulp and has lost other physical characteristics associated with the orlginal pulp. v t

In carrying out an acid sulphite digestion of kraft pulp with a liquor containing more combined than free S0 it is necessaryto permit a long period of contact between pulp and liquor in order toensure the desired reaction upon and removal of impurities associated with the pulp. Forinstance, if such digestion is carried out at boiling temperature and under atmospheric pressure, the timerequired may befrom six to ten hours, and even'if digestion is carried out at considerably higher temperatures and undersuper-atmospheric pressure, the time required may be from two to four hours. On the other hand, digestion of kraft pulp in an acid-sulphite liquor" containing more free than combined SO results in a removal of impurities in a much shorter time. Apparently, higher free than combined S0 content in the liquor is accompanied by a more rapid reaction upon and solution of impurities such as ligneous matter and pentosans from the fiber, the action prob-ably being analogous to that of a cooking liquor containing high free SO content upon raw cellulosic material, it being well known that a high free SO content in the cooking liquor is accompanied by a rapid liberation of fiber, as a result of rapid hydrolysis and solution of ligneous matter, pentosans, and other cementitious material. It is also known that the higher the free SO content of an acid-sulphite cooking liquor, the lower the temperature and pressure required for completing fiber liberation.

As previously stated, however, an acid-sulphite liquor containing more free than combined SO: degrades or injures a pulp markedly. For instance, assuming that one starts with kraft pulp having a pentosan content of 8% to 10%, a slowness of 10 minutes, a strength of 150, a tear resistance of 250, and a brown color, and digests such pulp at 212 F. as a 6% suspension in an acid sulphite liquor containing 2% free S0 and 1% combined S0 for two to three hours, the product will have av pentosan content of from 3% to 5%, a slowness of about 20 to 30 minutes, a strength of about 110 to 120, and a tear resistance of 140 to 160, and will be lighter in color. This product may be bleached to whiteness with considerably less bleach than is required for the original kr-aft pulp, but the bleached product has physical characteristics departing markedly from the original kraft pulp.

The present invention differs from the prior processes,.thus described, in that the kraft pulp, liberated and recovered from the wood by digestion in an alkaline working liquor, is subjected first, to digestion in an acid-sulphite liquor and second, to a digestion in an alkaline liquor, as a result of which certain benefits and advantages are secured as will be set forth. I have found, for example, that if kraft pulp which has been injured by digestion in an acidsulphite liquor, containing more free than combined S0 is then digested with an alkaline liquor to-remove prod nets of hydrolysis resulting from the acidsulphite treatment, the fiber is remarkably improved in physical and chemical characteristics, its strength and tear resistance being increased to values comparable with those of the original .pulp. The alpha cellulose content of the final product will be 91% to 95%, if sufficient alkali is present in the alkaline liquor. The resulting alkaline-digested pulp is readily bleachable to whiteness with a comparatively small amount of bleach, substantially without affecting its physical characteristics. The resulting final white product is accordingly an excellent raw material. for the manufacture of high grade papers and for use in the preparation of cellulose derivatives.

A kraft pulp which has been digested as hereinbefore described in an acid-sulphite liquor having more free than combined S0 may be washed and digested in alkaline liquors of various com ositions and under various temperature an pressure conditions. For example, the acid-sulphite-digested kraft pulp, after washing, may be digested as a 6% suspension in a caustic soda liquor containing from 8% to 12% NaOH, based on fiber, at 212 F. for about six hours, at the end of which time a finished product having the physical and chemical characteristics hereinbefore given is obtained. If desired, alkaline digestion may be carried out in closed digesters at higher temperatures, say at about 300 F.', in which case less time is required to produce the desired product. If'desired, a portion or all of the caustic soda may be replaced by alkalies such as sodium sulphide, sodium carbonate, or sodium sulphite, these milder alkalies preferably being used in high -temperature digestions, as they are then more effective than at lower temperatures. WVhen comparatively concentrated alkaline liquor is employed,- the acid-sulphite-digested kraft pulp maybe digested at comparatively low temperatures,for instance, room temperature. Thus, the washed, acid-sulphite-divgeste-d pulp may be digested as a 6% suspenwashed free of alkaline liquor, the resulting spent alkaline liquor preferably being reused, at least in part. particularly when it has a high content of unspent alkali. The

alkaline-digested pulp may be bleached to a pure white color with a consumption of only about 5% to 6% bleach, based on fiber, and finally washed.

The process of the present invention may likewise advantageously include a treatment of the initially liberated kraft pulp with one or more li'gnin-reactive reagents, such as chlorine water or bleach liquor, before the pulp is subjected to alkaline digestion, particularly when the pulp used as a raw ma terial is of an under-cooked or refractory variety and has a higher ligneous contentthan usual. Such a treatment may be effected upon the raw pulp, or after the pulp has been digested in an acid-sulphite liquor, and in such case the washing of the pulp after such treatment, and the final alkaline digestion, results in a removal of lignin reaction products and facilitates final bleaching to high whiteness without a physical or chemical degradation of the fiber. The chlorine water or bleach liquor apparently does not react appreciably upon the pentosans present in the pulp, but, as stated, reacts upon and removes lignin and allied substance, or renders such Substance soluble in an alkaline liquor. Specifically, the procedure may be to steep a raw pulp, such as kraft, for about two to three hours in chlorine Water at room temperature, containing, say, 3% to 4% chlorine, based on the weight of dry fiber, and then, preferably after Washing, to digest the pulp successively in an acid-sulphite liquor and in analkaline liquor, and finallyv to bleach the pulp as hereinbefore described. If desired, in lieu of treating with chlorine water, the pulp may be steeped in bleach liquor containing, say, 2% to 3% bleach, based on the dry weight of fiber, under the conditions hereinbefore given for the chlorine water treatment. Or the pulp may be treated, as described, successively with chlorine water and bleach liquor, or vice versa. Or either or both these treatments may be carried out after acid sulphite digestion rather than before such digestion. When bleach liquor is employed, it may be maintained distinctly alkaline during the treatment, by the addition of an'alkali such as caustic soda, to avoid formation of oxycelluloses.

While a kraft pulp which has been digested in an acid-sulphiteliquor containing more free than combined S0 may be improved to a greater extent by alkaline digestion than a pulp which has been digested in an acid-sulphite. liquor containing no more free than combined S0 (as in such first instance the pulp sufi'ers marked depreciation both physically and chemically), nevertheless, 'even in such second instance'it is, in certain cases,

advantageous to digest the sulphite-digested pulp in an alkaline liquor before bleaching,

' in order to refine the pulp to higher alpha cellulose content by the removal of reaction products of acid sulphite digestion and other non-alpha cellulose impurities. Consequently, While the presentprocess may bemore effectively practiced by a two-step treatment of kraft or soda pulp, in which the first or acid sulphite digestion is accomplished by a cooking liquor having the free S0 content higher than the combinedSO content, I do not confine myself thereto as the invention may be practiced by carrying on the acid sulphite digestion in a cooking, liquor in which the combined S0 is equal o or greater than the free S0 While the specific examples hereinbefore given deal specifically with kraft-pulp, the process of the present invention may also be applied to'advantage when soda pulp or other pulp liberated by the action of an alkaline liquor is used as a raw material. It will further be evident to those skilled in. the 'art that various other changes or modifications might be made in the process of the present invention Without departing from the spirit or scope of the appended claims. By the exmean S0 in excess of that necessary to form the acid sulphite or bisulphite of metals such as calcium, magnesium, sodium, or the like.

I claim: I

1. A process which comprises digesting cellulose pulp in an acid-sulphite liquor containing more free than combined S0 and the products of hydrolysis resulting from said first-mentioned digestion.

3. A process Whichcomprises digesting wood pulp, liberated by the action-of an alkaline liquor, in an acid-sulphite liquor containing more free than combined S0 to remove ligneous matter, pentosans, and other impurities from such pulp, then digesting the pulp in an alkaline liquor to remove the products of hydrolysis resulting from said first-mentioned digestion, and bleaching the alkaline-digested pulp.

41. A process which comprises digesting kraft pulp in an acid-sulphite liquor containing more free than combined S0 to remove ligneous matter, pentosa-ns, and other impurities from such pulp, Washing the pulp, digestingthe washed pulp in an alkaline liquor to remove the products of hydrolysis resulting-from said first-mentioned digestion,

.washing the alkaline-digested pulp, and' 7. A process which comprises steeping raw I pulp in a lignin-reactive reagent, digesting the pulp in ar. acid-sulphite liquor, digest- .ing the pulp in an alkaline liquor, and finally bleaching.

8. In a process which includes digesting wood pulp in an acid-sulphite liquor and then digesting the pulp in an alkaline liquor, that step before alkaline digestion which comprises treating the pulp with a ligninreactive reagent.

9. A process which comprises digesting wood pulp liberated by the action of alkaline liquor in an acid sulphite liquor, washing the pulp, digesting the washed pulp in an alkaline liquor, washing the alkaline-digested pulp, and finally bleaching in hypoehlorite bleach liquor.

In testimony whereof I have aflixed my signature.

GEORGE A. RICHTER. 

